Liquid-phase oxidation reactions are commonly employed for the oxidation of aldehydes to acids (e.g., propionaldehyde to propionic acid), the oxidation of cyclohexane to adipic acid, and the oxidation of alkyl aromatics to alcohols, acids, or diacids. One important example of the oxidation of alkyl aromatics is the liquid-phase catalytic oxidation of para-xylene to terephthalic acid which is a feedstock in the production of polyethylene terephthalate (“PET”). PET is a well-known plastic used in great quantities around the world to make products such as bottles, fibers, and packaging.
In a liquid-phase oxidation process, a liquid-phase feed stream and a gas-phase oxidant stream are introduced into a reactor and form a multi-phase reaction medium in the reactor. The liquid-phase feed stream introduced into the reactor contains at least one oxidizable organic compound, while the gas-phase oxidant stream contains molecular oxygen. At least a portion of the molecular oxygen introduced into the reactor as a gas dissolves into the liquid phase of the reaction medium to provide oxygen availability for the liquid-phase reaction. If the liquid phase of the multi-phase reaction medium contains an insufficient concentration of molecular oxygen (i.e., if certain portions of the reaction medium are “oxygen-starved”), undesirable side-reactions can generate impurities and/or the intended reactions can be retarded in rate.
If the liquid phase of the reaction medium contains too little of the oxidizable compound, the rate of reaction may be undesirably slow. Further, if the liquid phase of the reaction medium contains an excess concentration of the oxidizable compound, additional undesirable side-reactions can generate impurities. Therefore, in order to obtain product of high purity at minimum cost much effort is expended to achieve efficient utilization of raw materials, including recycle of raw materials, minimum energy consumption and minimal waste treatment cost.
In order to obtain uniform distribution of the multi-phase oxidation reaction medium liquid-phase oxidation reactors may be equipped with agitation means to promote dissolution of molecular oxygen into the liquid phase of the reaction medium, maintain relatively uniform concentrations of dissolved oxygen in the liquid phase of the reaction medium, and maintain relatively uniform concentrations of the oxidizable organic compound.
Thus, agitation of the reaction medium may be provided by mechanical agitation as provided in continuous stirred tank reactors (“CSTRs”). However, CSTRs have a relatively high capital cost due to their requirement for expensive motors, fluid-sealed bearings and drive shafts, and/or complex stirring mechanisms. Further, the rotating and/or oscillating mechanical components of conventional CSTRs require regular maintenance. The labor and shutdown time associated with such maintenance adds to the operating cost of CSTRs. However, even with regular maintenance, the mechanical agitation systems employed in CSTRs are prone to mechanical failure and may require replacement over relatively short periods of time.
Bubble column reactors provide agitation of the reaction medium without requiring expensive and unreliable mechanical equipment. Bubble column reactors typically include an elongated upright reaction zone within which the reaction medium is contained. Agitation of the reaction medium in the reaction zone is provided primarily by the natural buoyancy of gas bubbles rising through the liquid phase of the reaction medium. This natural-buoyancy agitation provided in bubble column reactors reduces capital and maintenance costs relative to mechanically agitated reactors. Further, the substantial absence of moving mechanical parts associated with bubble column reactors provide an oxidation system that is less prone to mechanical failure than mechanically agitated reactors.
The efficient manufacture of high volume oxidation products of interest, such as terephthalic acid involves not only the oxidation process but also the work-up and isolation of the product and multiple unit operations conducted in series or in parallel are often employed in the total manufacturing process. Each of these operations may require energy input or may be a source of energy which may be captured and utilized. Further, control of reaction variables and reactant stoichiometry to optimize product yield and purity while minimizing waste and environmental impact may be key to obtaining commercial success in the operation.
Terephthalic acid is conventionally produced by liquid-phase oxidation of para-xylene. In a typical process, a solvent liquid-phase feed stream and a gas-phase oxidant stream are introduced into a primary oxidation reactor with a catalyst system and form a multi-phase reaction medium in the reactor. The solvent present in the liquid-phase generally comprises a low molecular weight organic acid such as acetic acid and water. In production systems wherein the solvent is recycled, the solvent may contain small quantities of impurities such as, for example, para-tolualdehyde, terephthaldehyde, 4-carboxybenzaldehyde (4-CBA), benzoic acid, para-toluic acid, para-toluic aldehyde (4-methylbenzaldehyde), alpha-bromo-para-toluic acid, isophthalic acid, phthalic acid, trimellitic acid, polyaromatics, and/or suspended particulate.
The catalyst is a homogeneous, liquid-phase system comprising cobalt, bromine, and manganese.
As described above, the use of bubble column reactors for the primary oxidation reaction offers many advantages over conventional continuous stirred tank reactors, and oxidation processes employing bubble column reactors are disclosed, for example, in U.S. Pat. No. 7,355,068, U.S. Pat. No. 7,371,894, U.S. Pat. No. 7,568,361, U.S. Pat. No. 7,829,037, U.S. Pat. No. 7,910,769, U.S. Pat. No. 8,501,986, U.S. Pat. No. 8,685,334 and U.S. Pat. No. 8,790,601, the contents of which are hereby incorporated by reference. Bubble column reactors typically include an elongated upright reaction zone within which the reaction medium is contained and agitation of the reaction medium in the reaction zone is provided primarily by the natural buoyancy of gas bubbles rising through the liquid phase of the reaction medium. This natural-buoyancy agitation provided in bubble column reactors reduces utility power, capital, and maintenance costs relative to mechanically agitated reactors.
The initial oxidation reactor system may include both a primary oxidation reactor providing principally for oxidizing the majority of the liquid phase oxidizable compound and optionally at least one secondary oxidation reactor. The principal objective of this secondary oxidation, which is also referred to as Post Oxidation or as Early Oxidative Digestion as in U.S. Pat. No. 7,393,973 (hereby incorporated by reference in its entirety), is to oxidize a substantial fraction of the liquid phase aromatic oxidation intermediates from primary oxidation onwards to TPA before entering the more severe oxidation conditions of digestion. This provides a useful reduction in the total amount of over-oxidation to carbon oxides incurred subsequent to primary oxidation.
The product withdrawn from the primary oxidation system is typically a slurry comprising a particulate solid-phase of crude terephthalic acid (CTA) and a mother liquor. CTA contains relatively high levels of impurities (e.g., 4-carboxybenzaldehyde, para-toluic acid, fluorenones, and other color bodies) that render it unsuitable as a feedstock for the production of PET. Thus, the CTA is typically subjected to a purification process that converts the CTA particles into purified terephthalic acid (PTA) particles that may be suitable for production of polyethylene terephthalate. The further purification of CTA may include an oxidative digestion treatment subsequently followed by a hydrogenation treatment.
Typically a slurry of CTA particles in a mother liquor obtained from the primary oxidation system may contain from about 10 to about 50 weight percent solid CTA particles, with the balance being mainly the acetic acid mother liquor. The solid CTA particles present in the initial slurry withdrawn from primary oxidation system may contain from about 400 ppmw to about 15,000 ppmw of 4-carboxybenzaldehyde (4-CBA).
CTA may be converted to PTA by oxidative digestion treatment in a series of additional oxidation reactors commonly referred to as “digesters” wherein further oxidation reaction is conducted at slightly to significantly higher temperatures than were used in the primary and secondary oxidation reactors. Optionally, the slurry of CTA particles may be subjected to a solvent replacement step before proceeding to the digester units, whereby the replaced solvent has reduced concentrations of aromatic impurities and/or altered concentrations of catalyst and water that are readjusted to be more suitable for oxidation catalysis in the digester units. Optionally, the mass fraction of solids in the CTA slurry may also be adjusted, with or without solvent replacement, prior to entering the digester units.
In order to make the precipitated oxidation intermediate impurities available for oxidation in the series of digesters, the particles are exposed to higher temperatures than in the primary oxidation to at least partially dissolve the CTA particles and expose the impurities to liquid-phase oxidation comprising additional molecular oxygen injected into the digester. The high surface area, crystalline imperfections, and super-equilibrium impurity concentrations of the small CTA particles are favorable, both kinetically and thermodynamically, for partial dissolution and on-going recrystallization of the terephthalic acid when the CTA slurry temperature is raised moderately above the temperature at which the CTA was formed in the primary oxidation.
The further oxidation conducted in the digester system is intended to reduce the concentration of 4-CBA in the CTA particles. The digestion temperature may be from 5° C. to about 90° C. higher than the primary oxidation temperature and typically may be from about 150° C. to about 280° C.
In a second effect of the digestion process, the terephthalic acid particles may experience Ostwald ripening which tends to provide larger particles having a narrowed particle size distribution in comparison to the CTA particles in the outlet stream of the primary oxidation.
In a third effect of the digestion process, the recrystallized terephthalic acid particles comprise reduced concentrations of many of the impurities that are resistant to catalytic oxidative correction to form terephthalic acid, impurities such as polyaromatic carboxylic acid species, notably including many colored species such as 2,6-DCF and 2,7-DCF, inter alia. This reduction is caused by a closer approach to equilibrium distribution of the oxidation resistant impurities between solid and liquid phases, resulting from both the hotter operating temperature than in initial oxidation and also to the extended recrystallization time during the digestion process. The reduction in solid phase concentration of oxidation resistant impurities is further enhanced if the optional solvent replacement step has used a relatively more pure solvent such as, for example, distilled aqueous acetic acid from a solvent dehydration process used for removing the water produced by oxidation of the para-xylene.
Following the digestion treatment the purified product from the oxidative digestion may be crystallized and collected in one or more crystallization units and isolated by filtration to a mother liquor filtrate and a filter cake. The filtercake is extensively washed with solvent to remove catalyst and other impurities including methyl acetate formed during the oxidation processes.
The filtercake may then be blown free of retained wash and mother liquor and dried in an oven system to remove residual solvent.
A conventional hydrogenation process may then be employed for further purification. The dried washed filtercake is reslurried in water and catalytically hydrogenated to convert impurities to more desirable and/or easily-separable compounds.
The terephthalic acid may be selectively precipitated from the hydrogenated solution via multiple crystallization steps, and isolated.
The multiple systems required for primary oxidation, digestion, crystallization, filtration, drying and purification require energy management for thermal control of the various operations, overhead systems to manage exhaust vapors from the oxidation and digestion systems, a supply of air oxidant and systems to collect and recycle solvent and catalyst. Thus there is a need to further integrate existing liquid-phase oxidation systems for oxidation of dialkyl aromatic compounds to reduce total energy requirements while maximizing production efficiency and product yield. There is a further need to develop new liquid phase oxidation systems which provide efficient energy management, materials recycle and waste minimization in comparison to existing technology.